Split-Horizon DNS and SSL Certificate Validation : Why Your SSL Certificate Fails Despite Correct Installation

When an SSL Certificate fails to validate despite being correctly installed on your server, the cause often lies not with the SSL Certificate itself but with your network's Domain Name System (DNS) configuration.

Split-horizon Domain Name System (DNS), a common network architecture used by organizations to separate internal and external traffic, can create unexpected obstacles during SSL Certificate validation that leave website administrators puzzled and frustrated.

Understanding how split-horizon Domain Name System (DNS) interacts with SSL Certificate validation processes helps administrators diagnose validation failures, configure their networks appropriately, and ensure smooth SSL Certificate issuance and renewal.

This article explains what split-horizon Domain Name System (DNS) is, how it can interfere with SSL Certificate validation, and what steps you can take to resolve these issues.

What’s Split-Horizon Domain Name System (DNS)

Split-horizon Domain Name System (DNS), also known as split-brain Domain Name System (DNS) or split-view Domain Name System (DNS), is a network configuration technique where a Domain Name System (DNS) server provides different answers to the same query depending on where the request originates.

Organizations use this approach to serve internal users with private IP addresses while presenting external users with public-facing resources only.

This configuration creates two distinct views of your domain namespace that exist simultaneously.

The internal view contains Domain Name System (DNS) records pointing to private IP addresses, internal servers, and resources that should only be accessible within your corporate network.

The external view contains only publicly accessible services with their corresponding public IP addresses.

How Split-Horizon Domain Name System (DNS) Works

When a Domain Name System (DNS) query arrives at a split-horizon Domain Name System (DNS) server, the server examines the source IP address of the request. Based on whether the query originates from inside or outside the organization's network, the server consults different zone files to formulate its response.

For example, when an employee inside your corporate network queries "mail.yourcompany.com", they might receive the internal IP address 192.168.1.50, directing them to the mail server on your local network. When someone outside your network queries the same domain, they receive the public IP address 203.0.113.25, directing them through your firewall to the publicly accessible mail service.

This approach provides security benefits by hiding internal network structure from external observers. It also improves performance for internal users by directing them to local resources without routing traffic through external connections.

Common Uses for Split-Horizon Domain Name System (DNS)

Organizations implement split-horizon Domain Name System (DNS) for various operational and security reasons. Companies with internal web applications, intranets, or databases that employees access using the same domain names as public services benefit significantly from this configuration.

Businesses operating in regulated industries often use split-horizon Domain Name System (DNS) to maintain strict separation between internal and external network access. Healthcare organizations, financial institutions, and government agencies frequently employ this technique to protect sensitive internal resources.

Organizations with multiple office locations or hybrid cloud deployments use split-horizon Domain Name System (DNS) to direct users to geographically appropriate or network-appropriate resources while maintaining a unified external presence.

How SSL Certificate Validation Works

Before understanding how split-horizon Domain Name System (DNS) affects SSL Certificate validation, it helps to understand the validation process itself.

When you order an SSL Certificate, the Certificate Authority (CA) must verify that you control the domain for which you are requesting the SSL Certificate. This verification process uses several methods, all of which depend on proper Domain Name System (DNS) resolution.

The validation process occurs from the Certificate Authority (CA) perspective, meaning the Certificate Authority (CA) servers must be able to reach your domain and verify your control over it. This is where split-horizon Domain Name System (DNS) configurations can create problems.

HTTP-Based File Validation

HTTP-based validation, sometimes called file-based authentication, requires you to place a specific file with unique content at a designated path on your web server. The Certificate Authority (CA) then attempts to retrieve this file by making an HTTP or HTTPS request to your domain.

When the Certificate Authority (CA) queries your domain name to find your server's IP address, it receives the external view of your split-horizon Domain Name System (DNS) configuration. If your external Domain Name System (DNS) records point to a different server than where you placed the validation file, or if the external records are missing entirely, the validation fails.

Domain Name System (DNS) Based Validation

Domain Name System (DNS) based validation requires you to create a specific Domain Name System (DNS) record, typically a CNAME or TXT record, containing a unique validation token provided by the Certificate Authority (CA). The Certificate Authority (CA) then queries your Domain Name System (DNS) to verify the record exists.

With split-horizon Domain Name System (DNS), you might add the validation record to your internal Domain Name System (DNS) zone but forget to add it to your external zone. Since the Certificate Authority (CA) queries from outside your network, it receives responses from your external zone, which lacks the validation record, causing validation to fail.

E-Mail Based Validation

E-Mail based validation sends a verification e-mail to specific addresses associated with your domain, such as admin@yourdomain.com or hostmaster@yourdomain.com. While this method relies less directly on Domain Name System (DNS) resolution for the validation itself, the mail delivery still depends on proper MX records in your external Domain Name System (DNS) zone.

If your external MX records differ from your internal records or are missing, validation e-mails may not reach their intended recipients, causing validation delays or failures.

How Split-Horizon Domain Name System (DNS) Causes SSL Certificate Validation Failures

Split-horizon Domain Name System (DNS) creates SSL Certificate validation problems because Certificate Authorities (CAs) always query your domain from the external internet. They never see your internal Domain Name System (DNS) view, which means any configuration that works perfectly for internal users may be completely invisible to the Certificate Authority (CA).

Understanding the specific scenarios where problems occur helps administrators identify and resolve validation failures quickly.

Missing External Domain Name System (DNS) Records

The most common problem occurs when administrators add Domain Name System (DNS) validation records to their internal zone but neglect to add them to the external zone. This happens frequently because administrators often manage Domain Name System (DNS) from inside the network where they only see the internal view.

When you add a CNAME or TXT record for SSL Certificate validation to your internal Domain Name System (DNS) zone, queries from inside your network confirm the record exists. However, the Certificate Authority (CA) queries from outside your network and receives responses from your external zone, which contains no such record.

Similarly, if your internal Domain Name System (DNS) contains an A record pointing your domain to an internal IP address where you have placed a validation file, external queries may return a different IP address or no address at all.

Inconsistent Record Values

Sometimes both internal and external zones contain records for the same hostname, but with different values. Your internal A record might point to 192.168.1.100 where your development server runs, while your external A record points to 123.123.123.123 where your production server operates.

If you place the validation file on your development server because that is where your internal Domain Name System (DNS) directs you, the Certificate Authority (CA) attempts to retrieve the file from your production server and fails to find it.

Caching and Propagation Delays

Domain Name System (DNS) caching compounds split-horizon problems. When you add a validation record to your external zone, Domain Name System (DNS) servers around the internet may continue serving cached responses that lack your new record for the duration of the Time to Live (TTL) period.

If your internal Domain Name System (DNS) server has a shorter cache duration or you flushed its cache, internal queries show the new record immediately while external queries continue returning stale data. This creates confusing situations where you can verify the record exists from inside your network but the Certificate Authority (CA) cannot see it.

Firewall and Access Control Interactions

Split-horizon Domain Name System (DNS) often accompanies complex firewall configurations. Your external Domain Name System (DNS) might correctly point to a public IP address, but firewall rules might block the Certificate Authority (CA) from accessing the validation file or prevent Domain Name System (DNS) queries from reaching your authoritative name servers.

Some organizations configure their firewalls to block incoming connections except from known IP ranges. Since Certificate Authorities (CAs) use various IP addresses for validation checks, these restrictions can cause validation failures even when Domain Name System (DNS) configuration is correct. Explore Our SSL Certificate Validation Procedures 🔗

Diagnosing Split-Horizon Domain Name System (DNS) Validation Problems

When SSL Certificate validation fails unexpectedly, systematically checking for split-horizon Domain Name System (DNS) issues helps identify the root cause. Several diagnostic approaches reveal whether your Domain Name System (DNS) configuration is preventing successful validation.

Query Domain Name System (DNS) from Outside Your Network

The most effective diagnostic step involves querying your Domain Name System (DNS) from outside your corporate network, exactly as the Certificate Authority (CA) would. Use a mobile phone on cellular data, a home internet connection, or a cloud-based server to perform Domain Name System (DNS) lookups.

Online Domain Name System (DNS) lookup tools provide another option for viewing your domain as external users see it. These tools query public Domain Name System (DNS) servers and return the same results that Certificate Authorities (CAs) receive when validating your SSL Certificate.

Compare the results you see from outside your network with what you see from inside. If the records differ, you have identified a split-horizon configuration affecting your validation.

Verify Validation File Accessibility

For HTTP-based validation, test whether the validation file is accessible from outside your network. Use an external server or online tool to make an HTTP request to the validation URL. If you receive the correct file content from inside your network but an error or different content from outside, your split-horizon Domain Name System (DNS) is directing external traffic to a different server.

Some organizations maintain staging or development servers on internal IP addresses while production servers operate on external IP addresses. Ensure your validation file exists on the server that external Domain Name System (DNS) records reference.

Check Both Domain Name System (DNS) Zones

If you manage your own Domain Name System (DNS) servers, examine both your internal and external zone files directly. Verify that validation records exist in both zones and contain identical values. Pay particular attention to CNAME and TXT records used for Domain Name System (DNS) based validation.

For organizations using managed Domain Name System (DNS) services, you may need to access separate management interfaces for internal and external zones. Some cloud providers maintain these zones in different dashboards or require different credentials to access them.

Review Time to Live (TTL) Settings

Check the Time to Live (TTL) values on your Domain Name System (DNS) records. High TTL values mean changes propagate slowly across the internet. If you recently added validation records, external Domain Name System (DNS) servers may still be serving cached responses from before your changes.

Consider lowering TTL values temporarily before making validation-related Domain Name System (DNS) changes. This reduces the propagation delay and allows Certificate Authorities (CAs) to see your changes more quickly.

Resolving Split-Horizon Domain Name System (DNS) Validation Issues

Once you have identified that split-horizon Domain Name System (DNS) is causing your validation problems, several approaches can resolve the issue depending on your specific configuration and requirements.

Synchronize Internal and External Zones

The most straightforward solution involves ensuring that validation-related records exist in both your internal and external Domain Name System (DNS) zones with identical values. When you add a CNAME or TXT record for SSL Certificate validation, add it to both zones simultaneously.

For A records pointing to validation servers, ensure both zones reference servers accessible from their respective networks. Your internal zone can point to a private IP address while your external zone points to the public IP address of the same server, as long as the validation file exists on that server.

Establish procedures requiring Domain Name System (DNS) changes to be applied to both zones whenever the change affects SSL Certificate validation or public services.

Use Domain Name System (DNS) Based Validation Strategically

Domain Name System (DNS) based validation can be easier to manage in split-horizon environments because it requires only adding a record rather than ensuring file accessibility across network boundaries. However, you must still add the record to your external zone where the Certificate Authority (CA) can see it.

When using Domain Name System (DNS) validation, add the required CNAME or TXT record to your external authoritative Domain Name System (DNS) servers. You can optionally add it to your internal zone as well for consistency, but the external record is what the Certificate Authority (CA) verifies.

Configure Firewall Rules Appropriately

Review your firewall configuration to ensure Certificate Authority (CA) validation requests can reach your servers.

For HTTP-based validation, allow incoming HTTP and HTTPS connections to your web server from the internet, at least temporarily during validation.

For Domain Name System (DNS) based validation, ensure your authoritative Domain Name System (DNS) servers are accessible from the internet and can respond to queries from anywhere.

Consider Validation Method Alternatives

If your split-horizon Domain Name System (DNS) configuration creates persistent validation challenges, consider using e-mail based validation instead. E-Mail validation depends less on your Domain Name System (DNS) pointing directly to specific servers, though it still requires correct MX records in your external zone.

Best Practices for SSL Certificates in Split-Horizon Domain Name System (DNS) Environments

Organizations operating split-horizon Domain Name System (DNS) can implement several practices to ensure smooth SSL Certificate issuance and renewal while maintaining their desired network architecture.

Document Your Domain Name System (DNS) Architecture

Maintain clear documentation of your split-horizon Domain Name System (DNS) configuration, including which zones exist, where they are managed, and what differences exist between internal and external views. This documentation helps administrators understand the impact of changes and ensures consistency when multiple team members manage Domain Name System (DNS).

Include SSL Certificate validation requirements in your documentation, noting which records or files must be accessible externally for validation to succeed.

Establish Change Management Procedures

Create procedures that require Domain Name System (DNS) changes affecting public services to be applied to both internal and external zones. Include SSL Certificate validation in your change management process, ensuring administrators understand that validation occurs from outside the network.

When requesting new SSL Certificates or renewing existing ones, include verification steps that confirm validation resources are accessible externally before initiating the validation process.

Implement Automated SSL Certificate Management

Automated SSL Certificate management using the Automated Certificate Management Environment (ACME) protocol can simplify validation in split-horizon environments.

ACME clients handle the validation process automatically, but they must still be configured to create validation resources accessible from outside your network.

When configuring ACME clients in split-horizon environments, ensure the Domain Name System (DNS) plugin or HTTP challenge responder creates records or files that the Certificate Authority (CA) can access through your external Domain Name System (DNS) view.

Trustico® Certificate as a Service (CaaS) supports automated SSL Certificate management and can help organizations streamline their SSL Certificate lifecycle even in complex network environments. Explore Certificate as a Service (CaaS) 🔗

Test Validation Before SSL Certificate Expiry

Do not wait until your SSL Certificate is about to expire to discover validation problems. Test your validation configuration periodically by verifying that validation resources are accessible from outside your network.

Set up monitoring that alerts you to Domain Name System (DNS) configuration changes that might affect SSL Certificate validation. This proactive approach prevents last-minute validation failures that could result in SSL Certificate expiry and website downtime.

Special Considerations for Wildcard SSL Certificates

Wildcard SSL Certificates, which secure a domain and all its subdomains, present additional considerations in split-horizon Domain Name System (DNS) environments.

The validation process for Wildcard SSL Certificates typically requires Domain Name System (DNS) based validation rather than HTTP-based validation.

Domain Name System (DNS) Validation Requirements

Certificate Authorities (CAs) require Domain Name System (DNS) based validation for Wildcard SSL Certificates because HTTP-based validation cannot prove control over all possible subdomains. You must add the validation record to your external authoritative Domain Name System (DNS) zone where the Certificate Authority (CA) can verify it.

In split-horizon environments, this means specifically updating your external zone with the required CNAME or TXT record. Adding the record only to your internal zone results in validation failure because the Certificate Authority (CA) cannot see internal Domain Name System (DNS) records.

Subdomain Considerations

If your split-horizon configuration includes different subdomains in internal and external views, consider which subdomains you actually need to secure with your Wildcard SSL Certificate. The SSL Certificate secures subdomains at the Domain Name System (DNS) level, regardless of whether those subdomains resolve to internal or external IP addresses.

A Wildcard SSL Certificate for *.yourcompany.com works for any subdomain, whether it appears in your internal zone, external zone, or both. The key validation requirement is proving control over the base domain through external Domain Name System (DNS). Learn More About Wildcard SSL Certificates 🔗

Troubleshooting Common Split-Horizon Validation Scenarios

Several specific scenarios commonly cause validation problems in split-horizon Domain Name System (DNS) environments. Understanding these scenarios helps administrators quickly identify and resolve issues.

Validation Works Internally but Fails Externally

When you can verify the validation file or Domain Name System (DNS) record exists from inside your network but the Certificate Authority (CA) reports validation failure, your external Domain Name System (DNS) zone is missing the required records.

Add the validation records to your external zone or ensure your external A records point to the server containing the validation file.

Validation Worked Previously but Now Fails

If SSL Certificate renewal validation fails after previously succeeding, check whether Domain Name System (DNS) records have changed since your last validation. Someone may have modified external Domain Name System (DNS) records, or your Domain Name System (DNS) hosting provider may have made changes affecting your zone.

Also verify that any validation files are still in place on the server referenced by your external Domain Name System (DNS) records. Server migrations or configuration changes sometimes remove validation files without administrators realizing it.

Intermittent Validation Failures

Intermittent validation failures often indicate Domain Name System (DNS) propagation issues or load balancer configurations.

If your external Domain Name System (DNS) uses multiple servers or your web infrastructure uses load balancing, validation requests might reach different servers with inconsistent configurations.

Ensure all Domain Name System (DNS) servers serving your external zone contain identical records. If using load-balanced web servers, ensure the validation file exists on all servers in the pool.

SSL Certificates from Trustico®

Trustico® provides SSL Certificates with flexible validation options that work in various network configurations, including split-horizon Domain Name System (DNS) environments.

Our validation system supports HTTP-based, Domain Name System (DNS) based, and e-mail based validation methods, allowing you to choose the approach that best suits your infrastructure.

If you experience validation difficulties related to your network configuration, our support team can help diagnose the issue and recommend the most appropriate validation method for your environment. We understand that enterprise networks often involve complex Domain Name System (DNS) configurations, and we work with administrators to ensure successful SSL Certificate issuance.

Our SSL Certificate tracking system keeps you informed throughout the validation process, and our documentation provides detailed guidance on configuring validation for various scenarios. Explore Our SSL Certificate Support Resources 🔗

Frequently Asked Questions

Administrators managing split-horizon Domain Name System (DNS) environments commonly have questions about SSL Certificate validation in their configurations.

Can I Use HTTP Validation with Split-Horizon Domain Name System (DNS)?

Yes, HTTP validation works with split-horizon Domain Name System (DNS), but you must ensure the validation file is accessible via the server that your external Domain Name System (DNS) records reference.

If your internal Domain Name System (DNS) points to a different server than your external Domain Name System (DNS), place the validation file on the externally referenced server.

Why Does Validation Fail When I Can See the Record Internally?

Certificate Authorities (CAs) validate from outside your network and only see your external Domain Name System (DNS) view.

If you add validation records only to your internal zone, the Certificate Authority (CA) cannot see them. Always add validation records to your external authoritative Domain Name System (DNS) servers.

Do I Need to Disable Split-Horizon Domain Name System (DNS) for SSL Certificate Validation?

No, you do not need to disable split-horizon Domain Name System (DNS). You simply need to ensure that validation resources are accessible through your external Domain Name System (DNS) view.

This means adding validation Domain Name System (DNS) records to your external zone or ensuring external Domain Name System (DNS) records point to servers containing validation files.

How Long Should I Wait for Domain Name System (DNS) Changes to Propagate?

Domain Name System (DNS) propagation time depends on the Time to Live (TTL) values configured on your records.

Records with high TTL values may take hours or even days to propagate globally. Consider lowering TTL values before making validation-related changes, then raising them again after validation completes.

Which Validation Method Works Best with Split-Horizon Domain Name System (DNS)?

Domain Name System (DNS) based validation often works well because it requires only adding a record to your external zone without concerns about server accessibility.

However, any validation method works if properly configured. Choose the method that best fits your administrative processes and network architecture.